The present invention relates to the field of transmission by optical fiber. More particularly, the invention relates to xe2x80x9crepeaterlessxe2x80x9d links. Such links are defined by the fact that they use electrically active elements only in the end equipments.
Repeaterless optical fiber links have the particular feature of requiring very high optical power levels to be injected into the optical fiber in order to reach long distances. Two types of lightwave are injected into the fiber. The first type is the signal wave at about 1550 nm, which wave is modulated to carry the information that is to be transmitted. The second type of lightwave is a xe2x80x9cpumpxe2x80x9d lightwave in the 1400 nm to 1500 nm wavelength range, and is constituted by continuous power injected into the optical fiber so as to amplify the signal.
Numerous variants exist for using the pump wave. The pump wave can be emitted from the send terminal or from the receive terminal. The commonest scheme consists in placing a length of doped fiber in the link at a few tens of km from the terminal from which the pump wave is emitted. The doped fiber is activated by the pump lightwave and amplifies the signal. The pump wave can be injected into the same optical fiber as carries the signal, or it can be injected into a separate optical fiber. These two techniques can be combined. If the signal wave and the pump wave are in the same fiber, then the pump wave amplifies the signal by means of the Raman effect which, up to a certain level of pump power, is beneficial.
Attempts are being made to use very high signal powers and very high pump powers (of Watt order of magnitude) in order to increase the range of the link. However, the signal power and the pump power that can be injected are limited by various non-linear effects, and in particular the Brillouin effect, the Kerr effect, and the Raman effect. These effects are described in the work xe2x80x9cNon-linear fiber opticsxe2x80x9d, by G.P. Agrawal, Academic Press 1980. In such a repeaterless link, the Raman effect gives rise to a portion of the signal energy at 1550 nm being converted into noise around 1650 nm.
A. Hadjifotiou, in xe2x80x9cThe performance limits of unrepeated systemsxe2x80x9d, Suboptic ""93, Mar. 29-Apr. 2, 1993, Versailles, France, mentions limitation by various non-linear effects; for the Raman effect (or xe2x80x9cStimulated Raman Scatteringxe2x80x9d or xe2x80x9cSRSxe2x80x9d), that publication merely proposes a threshold power value on emission, corresponding to the emission power value at which the power that is frequency offset by the Raman effect (the Stokes power) is equal to the power of the signal at the outlet from the fiber.
The corresponding power level is greater than the power levels commonly transmitted over repeaterless links.
R.H. Stolen in xe2x80x9cPolarization effects in fiber Raman and Brillouin lasersxe2x80x9d, Journal of Quantum Electronics, Vol. QE-15, No. 10, October 1979, describes that Raman and Brillouin gains are twice as high in fibers when polarization is maintained. In typical long fibers which do not maintain linear polarization, Raman gain takes on a value which is about half the value it would have in the event of constant linear polarization. That article proposes an explanation and an experimental demonstration of that fact, for the purpose of obtaining high Raman gain in lasers.
GB-A-2 307 368 describes a polarization scrambler for a wavelength division multiplexed (WDM) transmission system with repeaters. In that system, it is proposed that steps should be taken to ensure that adjacent wavelengths in the comb of wavelengths have polarizations that are orthogonal, or at least different, so as to diminish the four-wave mixing effects between signals at adjacent wavelengths.
As a result, in the state of the art, the powers injected into repeaterless links are less than the threshold power for the Raman effect. This effect is therefore not generally perceived as being penalizing.
The invention proposes a solution that makes it possible to increase power in a repeaterless link. It goes against the general prejudice in the state of the art by proposing to use inlet power into a repeaterless system in excess of the power threshold value for the Raman effect, and which approaches the power limit of the fiber.
More precisely, the invention proposes a repeaterless optical fiber transmission system having polarization scrambling means for scrambling the polarization of the lightwaves injected into the system.
In an embodiment, the polarization scrambling means scramble the polarization of the information-carrying waves.
Preferably, the polarization scrambling means scramble the polarization of the pump waves.
Advantageously, the polarization scrambling means scramble polarization in the time domain.
In which case, the polarization scrambling means comprise a high frequency polarization scrambler.
In another embodiment, the polarization scrambling means comprise a low frequency polarization scrambler.
Advantageously, the polarization scrambling means scramble polarization in the space domain.
In which case, the polarization scrambling means comprise a passive birefringent device.